The objective of this Program is to design and develop three classes of highly novel HIV-1 vaccine immunogens based on the critical viral envelope (Env) protein, and subsequently test these vaccine candidates to determine their capacity to elicit broadly neutralizing antibody responses required to provide protection from HIV infection. Vesicular stomatitis virus (VSV) vectors take on strategic roles in design, development and delivery of these new vaccine candidates. The three new immunogen platforms are designed specifically for expression by VSV vectors as transmembrane proteins that will present epitopes to the immune system in several natural contexts including;on the surfaces of vector particles in the vaccine, in the membrane of infected cells following vaccination, and on the surface of progeny virus particles produced in the vaccinated host. The novel immunogen platforms that will be developed include: i) stable Env trimers optimized for expression by VSV;ii) Env epitopes derived from the membrane-proximal external region (MPER) displayed with a VSV G carrier molecule or scaffold;and iii) MPER epitopes presented in a membrane- proximal environment with a truncated G protein platform called G-Stem. Moreover, a novel process will be developed that makes use of the innate and dynamic ability of VSV to genetically adapt, through accrual of mutations, to biologically derived unique immunogen configurations, which will exhibit enhanced characteristics such as improved expression, greater abundance in viral and cellular membranes, increased stability, or critical epitope exposure and conformation. Experimental vaccines will be tested, characterized and ranked 'in vitro'before top candidates are advanced into rabbit immunogenicity studies to assess the character of the humoral immune response directed by the new vaccines. Candidate vaccines that elicit broadly neutralizing antibody responses in rabbits will be evaluated in a macaque challenge protection model. Antibody responses in vaccinated macaques will be evaluated thoroughly, and then animals will be challenged with a pathogenic hybrid simian/human immunodeficiency virus (SHIV) to assess vaccine efficacy. Project Relevance: Three new experimental vaccines will be developed specifically to stimulate the immune system to produce antibody responses against HIV-1. These vaccines will be developed using a novel methodology to biologically evolve candidates that elicit robust immune responses against specific regions of the HIV envelope protein. Promising vaccine candidates will be evaluated systematically in animal models to determine whether they elicit antibody responses that are more efficacious than those produced with earlier experimental vaccines.